AW additive

Encyclopedia : A : AW : AWA : AW additive

AW additives, or antiwear additives, are additives for lubricants with role to prevent metal-to-metal contact between parts of the gears.

EP additives are usually used in places like gearboxes, while AW additives are used in places with lighter load like bushings.

AW additives typically contain zinc and phosphorus compounds.

Some popular AW additives are:

zinc dithiophosphate (ZDP)
zinc dialkyl dithio phosphate (ZDDP), probably the most widely used in formulated engine oils, also acts as a corrosion inhibitor and antioxidant
Tricresyl phosphate (TCP), used for high-temperature operation, often used as an AW and EP additive in turbine engine lubricants, and also in some crankcase oils and hydraulic fluids
Halocarbons (chlorinated paraffins), for extreme pressure operations
Glycerol mono oleate
Stearic acid, adhering to the surface via reversible adsorption process at under 150 °C, which limits its efficiency to only mild contact conditions.

Some formulations use colloidal PTFE (Teflon), but its efficiency is controversial.

Many AW additives double as EP additives, eg. various organophosphates or sulfur compounds. The mechanism of function of TCP and ZDDP is explained in EP additives.

Under extreme pressure conditions, the performance of AW additives becomes insufficient and designated EP additives are required.